1. Field of the Invention
This invention relates to an apparatus for controlling the air/fuel ratio of an internal combustion engine.
2. Description of the Related Art
An exhaust gas purifying system is conventionally known wherein a three-way catalyst for purifying exhaust gas of an internal combustion engine is disposed in an exhaust system of the internal combustion engine to purify exhaust gas of the engine.
It is already known that the exhaust gas purifying efficiency of such an exhaust gas purifying system can be improved by fluctuating the air/fuel ratio around the theoretical air/fuel ratio.
To this end, an oxygen concentration sensor of the .lambda. type (which denotes an oxygen concentration sensor which presents a sudden change in output value thereof around a predetermined air/fuel ratio (theoretical air/fuel ratio, and such sensor will be hereinafter referred to as O.sub.2 sensor) is conventionally provided in an exhaust manifold, i.e., on an upstream side of a catalytic converter. Interested with the fact that the output of such O.sub.2 sensor presents a change from an on-state to an off-state, that is, a change from a high voltage level to a low voltage level or vice versa across the predetermined air/fuel ratio (theoretical air/fuel ratio), the output of the O.sub.2 sensor is fed back to control the air/fuel ratio so that the air/fuel ratio may remain around the theoretical air/fuel ratio. Such control is called O.sub.2 feedback control.
In such O.sub.2 feedback control, an output of the O.sub.2 sensor is compared with an on/off threshold voltage (reference value), and if, for example, the O.sub.2 sensor output is higher than the threshold voltage, the air/fuel ratio is controlled toward the lean side, but on the contrary, if the O.sub.2 sensor output is lower than the threshold voltage, the air/fuel ratio is controlled toward the rich side.
With such conventional O.sub.2 feedback control, however, there is the possibility that, if the O.sub.2 sensor used for the feedback control undergoes a secular change or deterioration, the reliability of control may be deteriorated. Further, quality, in particular, sensitivity dispersion of O.sub.2 sensors leads to large dispersion of emission levels. This may also result in a reduction to the reliability of control.
Further, since the maximum frequency of variations in air/fuel ratio is restricted by a delay (waste time) in conveyance of gas from a fuel supply station to the location of the O.sub.2 sensor as well as a delay in the response by the sensor, there is the possibility that the capacity of the catalyzer may not be exhibited sufficiently.
Means has thus been proposed for further improving the exhaust gas purifying characteristic of an exhaust gas purifying system of an internal combustion engine. Such means is disclosed, for example, in Japanese Patent Laid-Open No. 56-118535 wherein the air/fuel ratio of an air/fuel mixture to be introduced into a three-way catalyst is changed positively.
With such conventional means, however, since the median of variations of the air/fuel ratio is invariable, there still is the possibility that the air/fuel ratio cannot be changed around the maximum purifying efficiency of the three-way catalyst.
It may hence be contemplated of arranging an O.sub.2 sensor on a upstream or downstream side of a catalytic converter to control the compulsorily changed state of the air/fuel ratio, such as the average of variations in the air/fuel ratio (the average air/fuel ratio) on the basis of the results of a comparison between an output from the O.sub.2 sensor and a target value corresponding to a desired air/fuel ratio. Such means however involves the following problems when the output of the O.sub.2 sensor indicates a rich air/fuel ratio as a result of control by the O.sub.2 sensor and the timing of acceleration, for example, in a small intake-air-quantity operation state (low-speed and low-load operation state, low-load operation state, idling state, or the like) before acceleration [see FIG. 19(a), point al]. Since the catalytic converter is in an oxygendeficient state before such acceleration, acceleration
FIG. 19(c) in such a state leads to the problem that the emission of HC and CO increases immediately after the acceleration [see the characteristic curve shown by a solid line in FIG. 19(a)]. In addition, the catalytic converter is brought into an oxygen-excessive lean state because of the control by the O.sub.2 sensor after the acceleration [see FIG. 19(a), point a2]. This results in a reduction to the efficiency of purification of NOx, so that more NOx is emitted as shown by the dashed characteristic curve in FIG. 19(b).